Ionic liquids are salts that are liquid at ambient or near ambient temperatures. Ionic liquids have a number of uses which include replacing organic solvents in chemical processes and reactions, extracting organic compounds from aqueous waste streams, and as electrolytes in devices such as capacitors and batteries. This is because, unlike conventional organic solvents, ionic liquids are non-volatile and non-flammable. These properties are advantageous to help reduce losses to evaporation, eliminate volatile organic emissions, and improve safety.
Other properties of ionic liquids have also proved advantageous. For example, many ionic liquids have a broad temperature range at which they remain liquid and also are stable over a broad pH range. This is beneficial for high temperature processes with a demanding pH. Further, some ionic liquid systems can be used as both a solvent and catalyst. For example, [bmim]-Al2Cl7 and [emim]-Al2Cl7 can be employed as a solvent and catalyst in Friedel-Crafts reactions wherein bmim is 1-butyl-3methylimidazolium and emim is 1-ethyl-3-methylimidazolium.
Ionic liquids are conventionally prepared by metathesis, i.e., double decomposition reaction, whereby the reaction of two compounds forms two new compounds—one of which is the ionic liquid. For example, reacting [emim]Cl with AgNO3 will yield [emim]NO3 and AgCl.
Unfortunately, such metathesis reactions produce a mole of waste, e.g., one mole of AgCl per mole of product. In addition, the ionic liquids often contain impurities from incomplete reactions and solvent contamination. A further problem with metathesis is that it requires the separation of the two compounds which are produced. While this can be accomplished readily when the ionic liquid is hydrophobic, it is more difficult when the ionic liquid is hydrophilic. In addition, the required reagents (e.g., AgNO3) are expensive and difficult to recycle.
For the aforementioned reasons, it would be desirable to discover a new process for preparing ionic liquids. It would further be desirable if such a process was capable of making both hydrophobic and hydrophilic ionic liquids with low amounts of waste and impurities and without the use of an organic solvent.
Advantageously, new processes have been discovered to make ionic liquids. The processes comprise using an electrochemical cell which comprises an anode and a cathode. The cell is charged with solutions and subjected to electrolysis to produce the desired ionic liquid. The ionic liquid is then recovered. Alternatively, a precursor to the ionic liquid is made in an electrochemical cell. The precursor is then converted, chemically or otherwise, to the desired ionic liquid. The instant inventive processes can produce ionic liquids having a purity of 99% or higher.